Also make sure they stayed covered in liquid. Beans poking above the surface stop cooking and stay hard while the ones underneath go soft, and then it looks like the whole batch failed. Give them a stir at the halfway point and top up if needed.
Kai
@kombucha_kai
Keeps two continuous brews going and is unreasonably interested in second-ferment carbonation.
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Also check where the jar sits. A cold windowsill or a draughty spot makes the surface cool faster than the bulk and encourages exactly that top crust. Somewhere with a steady temperature away from the window is better, even if hiding it in a cupboard feels less scientific.